The storage and transmission of full-color, full-motion images is increasingly in demand. These images are used, not only for entertainment, as in motion picture or television productions, but also for analytical and diagnostic tasks such as engineering analysis and medical imaging.
There are several advantages to providing these images in digital form. The images are more susceptible to enhancement and manipulation. As with all digital signals, digital video images can be regenerated accurately over several generations with only minimal signal degradation.
On the other hand, digital video requires significant memory capacity for storage and, equivalently, it requires a high-bandwidth channel for transmission. For example, a single 512 by 512 pixel gray-scale image with 256 gray levels requires more than 256,000 bytes of storage. A full color image requires nearly 800,000 bytes. Natural-looking motion requires that images be updated at least 30 times per second. A transmission channel for natural-looking full color moving images must therefore accommodate approximately 190 million bits per second. One minute of full color video requires almost 2 gigabytes of storage.
As a result, a number of image compression techniques have been proposed to reduce the information capacity required for storage and transmission of digital video signals. These techniques generally take advantage of the considerable redundancy in any natural image and the limits of the human psycho-visual system which does not respond to abrupt time-based or spatial transitions. Both time-domain and spatial-domain techniques are used to reduce the amount of data used to transmit, record, and reproduce color digital video images.
For example, differential pulse-code modulation (DPCM) is a commonly-used compression technique which relies upon the facts that video images, generally, are quite redundant and that any transitions in the images are, for the most part, gradual. A DPCM encoder predicts each pixel value from previous pixel values. It then compares the actual value with the predicted value to obtain an error signal. The error is the encoded value. If the predictions are relatively accurate, the error will be small and its value will occupy a great deal less memory and/or bandwidth than the original video signal. The signal can be decoded by using the prediction algorithm in conjunction with the error signal.
A color image may be represented as a combination of luminance and color-difference signals. For example, a digitized color image may have one byte assigned for the color difference signal for each pixel. This image information may be compressed by recognizing that the human psycho-visual system is limited in its ability to detect subtle variations in color and therefore assigning a single chrominance value to a group of neighboring pixels which are of approximately the same color.
A digitized image's luminance information can also be encoded for compression, but, because the human psycho-visual system is more sensitive to luminance changes than to color changes, greater care must be taken in reducing luminance information. A number of the advantages of digital television, in addition to a number of data compression techniques, are discussed in chapters 18 and 19 of "Television Engineering Handbook", K Blair Benson, Editor in Chief, McGraw-Hill Book Company, 1986 which is hereby incorporated by reference.
A commonly-employed method of luminance reduction is block truncation coding. This technique entails dividing an image into contiguous, non-overlapping regions, then encoding the luminance of each of the regions using two luminance values and a bit mask. The bit mask indicates which of two luminance values is to be assigned to a particular pixel within the region. If the regions are too large, the image takes on a "contoured" look, thus degrading the image quality. On the other hand, if the regions are too small, very little image compression is achieved.
It is therefore an object of the invention to reduce the data required to represent the luminance information of an image which is represented in a luminance and color-difference format. It is a further object of the invention to improve the quality of a compressed image. A further object of the invention is to improve the degree of compression that can be achieved for a given level of image quality. A still further object of the invention is to permit interactive tradeoffs between the degree of compression and the quality of an image.